1. Field of the Invention
The present invention relates to a radiation image storage panel employable in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of Prior Art
As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been proposed and practically used. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The radiation image storage panel is generally used repeatedly, after the recorded image is erased.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and a radiographic intensifying screen. Further, the method is very advantageous from the viewpoints of conservation of resources and economical efficiency, because the radiation image storage panel can be repeatedly used in the method, while the radiographic film is consumed in each radiographic process in the conventional radiography.
Moreover, the radiation image recording and reproducing method using a stimulable phosphor is of great value especially when the method is employed for medical diagnosis, because a radiation image can be obtained in the method with a sufficient amount of information by applying a radiation to an object at a small dose as described above.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. If the phosphor layer is self-supporting, the support may be omitted. Further, a transparent film of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer generally comprises a binder and a stimulable phosphor dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore. It is desired for the radiation image storage panel employed in the method to have a high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.). The radiation image storage panel is repeatedly used, and therefore the panel is further desired to be resistant to physical shocks and environmental variations (variations of temperature, humidity, etc.), from the viewpoints of reliability of the obtained image data, economical efficiency and easiness of handling.
The sensitivity of the radiation image storage panel is essentially determined by the total amount of stimulated emission given by the stimulable phosphor contained therein, and the total emission amount varies depending upon not only the emission luminance of the phosphorut also the content (i.e, amount) of the phosphor in the phosphor layer. The large content of the phosphor also results in increase of absorption of a radiation such as X-rays, so that the panel shows an increased high sensitivity and provides an image of improved quality, especially graininess. On the other hand, assuming that the content of the phosphor in the phosphor layer is kept at the same level, a panel utilizing such a phosphor layer provides an image of high sharpness if the phosphor layer is densely packed with the phosphor, because such phosphor layer can be made thinner to reduce spread of stimulating rays caused by scattering in the phosphor layer.
The phosphor layer is generally prepared by coating a phosphor dispersion comprising stimulable phosphor particles and a binder in an appropriate solvent over a support or a sheet using a known coating means such as a doctor blade or a roll coater and drying the coated layer. Thus prepared phosphor layer comprising a binder and a stimulable phosphor dispersed therein has a certain upper limit with respect to amount of a phosphor incorporatable therein or the density of the phosphor, so that the panel having such phosphor layer cannot show a sensitivity beyond a certain limit or cannot provide an image of sufficiently high quality beyond a certain limit.
There is also known a phosphor layer comprising an agglomerate of a stimulable phosphor other than the above phosphor layer composed of phosphor particles dispersed in a binder.
There have been known methods for forming a phosphor layer which contains no binder and consists of only a stimulable phosphor. For instance, U.S. Pat. No. 3,859,527 teaches a temporary storage medium using a hot pressed phosphor, and Japanese Patent Provisional Publication No. 61(1986)-73100 describes a phosphor layer formed by a firing process.
The present inventor has already applied for patent with respect to a radiation image storage panel comprising a support and a phosphor layer which comprises a sintered stimulable phosphor and a process for the preparation of said panel (U.S. patent application No. 072,698).
The present inventor has also applied for patent with respect to a radiation image storage panel having a phosphor layer of a sintered stimulable phosphor or a deposited stimulable phosphor which is impregnated with a polymer material and a process for the preparation of said panel (U.S. patent application No. 184,010).
The phosphor layer composed of an agglomerate of a stimulable phosphor or a deposited stimulable phosphor can be formed utilizing a sintering method, a deposition method, etc., and the phosphor layer impregnated with a polymer material can be formed by first preparing a phosphor layer of the sintered stimulable phosphor or the deposited stimulable phosphor not containing a polymer material, and then incorporating a polymer material into the phosphor layer.
In those phosphor layers, the phosphor particles are not dispersed in a binder but agglomerated to be in close contact with each other in the absence of a binder. A polymer material if it is incorporated in the aggromerated phosphor layer is present only within voids of the formed agglomerate of the stimulable phosphor (e.g., between boundary portions of the phosphor particles and/or within pore portions of the phosphor particles).
As stated hereinbefore, the radiation image storage panel is desired to be well resistant to physical shocks and environmental variations (variations of temperature, humidity, etc.), to have high sensitivity and to provide an image of high quality. Particularly, a panel having a phosphor layer composed of only an agglomerate of a stimulable phosphor is deformed or placed under strain caused by difference of thermal expansion rate between the phosphor layer and the support, and the deformation or strain gives a stress to the phosphor layer and the support. As a result, the phosphor layer is apt to be cracked or the support is easily distorted. Further, when a physical shock is applied to the support of the panel for example by dropping the panel, the shock is transferred to the phosphor layer to cause cracks of the phosphor layer.
The above-described unfavorable feature can be improved to a certain extent by impregnating a polymer material into the phosphor layer, but the improvement is still not satisfactory.